The treatment of wastewater requires the removal of organic and inorganic contaminants present in solid and/or dissolved form. Organic contaminants include proteins, lipids and polysaccharides as well as hazardous compounds such as aromatic and aliphatic hydrocarbons. Nitrogen-based and phosphorus-based compounds, which have been recognized as major contributors to eutrophication, also need to be removed during the treatment process.
Biological wastewater treatment systems generally use a variety of microorganisms for efficient and complete biodegradation of contaminating compounds. The organic charge in the wastewater is often measured by biochemical oxygen demand (BOD), which defines the overall oxygen load that a wastewater will impose on receiving water. During biological wastewater treatment, organic substances, and some nitrogen-based and phosphorus-based compounds are consumed by microorganisms as essential nutrients to support microbial growth during assimilatory processes. Excess amounts of nitrogen-based compounds also are removed during dissimilatory microbial nitrogen metabolism, during which they are transformed to molecular nitrogen and released into the atmosphere. Specifically, the microbial consortia within the wastewater treatment system first converts ammonia to nitrate (nitirification) under oxic conditions, and then convert the nitrate to molecular nitrogen (denitrification) under anoxic conditions. Excess phosphorus-based compounds may be removed under by the “luxury phosphorus uptake” process where some microbes within the wastewater treatment system accumulate phosphorus and store it as poly-phosphorus compounds, thus removing it from the system during sludge disposal.
Biological wastewater treatment systems capable of removing nitrogen and phosphorus generally include alternating anoxic and oxic environments, such as alternating tanks or zones configured to receive different amounts of air (i.e., oxygen) via blowers. The effectiveness of such systems depends on the ability to control the amount of dissolved oxygen within each environment, while still providing for adequate biomass growth, maintenance of different microbial species, effective solid-liquid separation, sludge stabilization, and proper optimization and control of environmental conditions in the multiple zones of the treatment system. Existing systems have attempted to achieve the best conditions for nitrogen removal, but generally introduce too much dissolved oxygen into the various zones (thereby inhibiting denitirification and phosphorous removal), and often require substantial amounts of energy, mechanical equipment and/or physical space.